1. Field of the Invention
This invention relates to a method of stressing concrete reinforcement elements and subsequently anchoring them in the stressed condition, and also to a hydraulic jack for stressing simultaneously a plurality of concrete reinforcement elements and for anchoring the stressed elements. The invention is applicable both to pre-stressing of elements in a mould for a concrete structure, and for post-stressing of elements in a preformed structure.
2. Description of the Prior Art
In recent years, there have come into use hydraulic jacks which are capable of stressing simultaneously a large number of individual reinforcing elements, for instance as many as 150. The elements may each be for instance an individual wire or strand (i.e. a plurality of individual wires twisted together) or a group of wires not twisted together. Usually, such jacks have a gripping plate having a bore for each element which provides a conical seat for split conical gripping wedges which grip the element while the gripping plate is moved rearwardly, to stress the elements simultaneously. A second function which the jacks perform is to anchor the elements after stressing, in an anchoring plate which abuts the end of the concrete structure or is a part of the mould. Before stressing, split conical anchoring wedges are arranged in conical seats in the bores in the anchoring plate through which the ends of the elements project. The jack has a lock-off piston (also called the anchoring piston) which is actuated after stressing to cause the anchoring wedges to be urged into their seats to grip the elements. The jack itself can then be released and removed. It is usual to provide a lock-off plate which is driven forward by the lock-off piston so as to engage and drive home the anchoring wedges.
In past proposals, the jack has been supported during stressing on the anchoring plate, through which it transmits the load to the concrete (in post-stressing) or the mould (in pre-stressing). This is shown for example in British patent specification No. 1,143,641 and U.S. Pat. No. 3,610,581. The difficulty then arises that the anchoring plate may be mis-aligned with respect to the position which the reinforcement elements naturally wish to take upon being stressed. A development therefore has been to support the jack on a ring surrounding the anchoring plate, and to allow the anchoring plate a very small degree of movement with respect to the jack, so that it may align itself correctly. This is shown in British patent specification No. 1,402,655 and corresponding to U.S. Pat. No. 3,844,023. In British patent specification No. 1,151,708 a small amount of axial movement of the anchoring plate is also permitted, to allow the insertion of a spacer plate between the anchoring plate and its abutment.
Even if the anchoring plate is allowed to align itself, a considerable amount of frictional drag is experienced by the elements as they are pulled through the bores in the anchoring plate, because for example the elements extend parallel to each other inside the jack, whereas they converge towards each other as they extend away from the anchoring plate through the concrete or mould. In this way a bend is formed in the elements at the anchoring plate. This frictional drag is present throughout the stressing stroke and can considerably reduce the total load which the jack can impart to the stressed tendons.